1. Field of the Invention
The present invention is generally related to rotary position sensors and, more particularly, to a throttle position sensor which maintains a constant gap between a rotatable magnet and a magnetically sensitive transducer even if the rotational axis of the throttle shaft may move from its original position.
2. Description of the Prior Art
Many different types of position sensors are well known to those skilled in the art. Some position sensors are intended for use in the determination of the angular position of a rotatable shaft. For example, U.S. Pat. No. 4,257,040, which issued to Shirasaki et al on Mar. 17, 1981, discloses an engine rotational information detecting apparatus that provides information which is indicative of the rotation of an engine. A multiplicity of magnetic poles are formed on a track along the circumfrential periphery of a rotating member driven by an engine. This permits the rotational reference position to be detected and a rotational angle to be determined from the rotational reference position of the engine. A reference sensor and an angle sensor are provided for generating a signal representing a change in magnetic flux by detection of the magnetic flux from the track. Some of the multiplicity of magnetic poles on the track are opposed to the rotational reference sensor and are omitted so that a signal representing the rotational reference position is generated in the case where the angle sensor detects magnetic flux, but the rotational reference sensor fails to detect the magnetic flux.
U.S. Pat. No. 3,893,059, which issued to Nowak on Jul. 1, 1975, discloses a pulse generator with an asymmetrical multiple pole magnet. The generator comprises an asymmetrical magnet assembly and a pulse sensor that is operative for generating an electrical signal upon the application of a magnetic field thereto. The sensor and the magnet assembly are mounted for relative movement for the generation of a train of electrical signals in response to relative displacement therebetween. The asymmetrical multiple pole magnets in the magnet assembly have a plurality of asymmetrically spaced pole pairs with each pole pair including a magnetic pole of predominate plurality occupying a substantially greater area than the pole of opposite plurality.
U.S. Pat. No. 3,942,045, which issued to Palazzetti on Mar. 2, 1976, describes a speed or angle position electromagnetic transducer which comprises a permanent magnet with closely spaced pole faces and a ferromagnetic core magnetically connected to the magnet and carrying a coil. The transducer cooperates with a toothed wheel the teeth of which are moved past the pole faces and the core in such a way that two magnetic circuits are completed by the teeth, one passing through the teeth and the two pole faces and the other passing through the teeth and the core.
U.S. Pat. No. 3,835,373, which issued to Matula on Sep. 10, 1974, discloses a rotational position sensor that utilizes a Hall device and a means to maintain the Hall voltage constant. The Hall effect device is mounted within an air gap of variable size within a magnetic circuit. The Hall effect device is subjected to changes in magnetic flu density in direct relation to the rotational position of a cylindrical member forming a part of the magnetic circuit. The magnetic circuit includes a C-shaped permanent magnet having opposite pole pieces forming air gaps with the cylindrical member. The cylindrical member comprises a half cylinder in the region of the air gap containing a Hall effect device so as to vary the effective area of the gap and thus the flux density as the cylindrical member turns.
U.S. Pat. No. 3,818,326, which issued to Masuda et al on Jun. 18, 1974, discloses a rotary sensor that uses magnets for detecting the rotation of a mechanical system. The rotary sensor comprises a frame member having at least two bar yokes which are magnetically connected by a connecting yoke and a shaft member which is rotated in a space formed between the bar yokes, wherein said shaft member is provided with at least two segment yokes fixed in sequence in the lengthwise direction of the shaft. The segment yokes are arranged so that their rotating ends are deviated from each other and the bar yokes are provided with Galvano-magneto effect devices so that the devices approach the rotating ends of the segment yokes when the segment yokes are rotated.
As is well known to those skilled in the art, the output signal from a magnetically sensitive device, such as a Hall effect transducer, varies as a function of the strength and direction of a magnetic field imposed on the magnetically sensitive device. If the source of a magnetic field, such as a magnet, is moved toward the magnetically sensitive device or away from it, the strength of the magnetic field imposed on the device changes. Therefore, if a shaft position is sensed by a stationary magnetically sensitive device disposed proximate a magnet attached to the shaft, movement of the rotational axis of the shaft toward or away from the magnetically sensitive device will change the effective strength of the magnetic field imposed on the device. This change in magnetic field strength at the position of the magnetically sensitive device will therefore change the magnitude of an output signal provided by the magnetically sensitive device since the output signal is representative of the magnetic field strength imposed on the magnetically sensitive device.
This problem exists in systems for determining the angular position of a throttle shaft so that the position of a throttle plate can be determined. If the magnetically sensitive device is mounted rigidly to a stationary member and a magnet is attached to the throttle shaft, movement of the throttle shaft toward or away from the magnetically sensitive device will create inconsistent magnetic field strengths which will not accurately indicate the angular position of the throttle plate shaft and will therefore result in incorrect output signals from the magnetically sensitive device. The change in distance between the magnet and the magnetically sensitive device can be caused by wear in the bearings supporting the throttle plate shaft. In addition, variation in this critical distance between the magnet and the magnetically sensitive device can result from wobble of the throttle plate shaft as it rotates.
It would therefore be beneficial if a means for sensing the angular position of a rotatable shaft could avoid the adverse affects of bearing wear or shaft wobble.